I. FIELD OF THE INVENTION
The present invention relates generally to earth boring drill bits having rotatable rolling cutters mounted thereon, and more specifically to the positioning of wear resistant inserts located on the gage rows of the cutters.
II. DESCRIPTION OF THE PRIOR ART
Earth boring bits for drilling oil and gas wells typically have three rotatable cutters that roll over the bottom of a borehole as the bit rotates. Each cutter is generally conical and has a frustoconical heel surface that passes near the borehole sidewall as the cutter rotates. One type of bit, known as a tungsten carbide insert bit or TCI bit, has wear resistant inserts secured in holes formed in the cutters. Such inserts are usually made of tungsten carbide.
For each cutter, the inserts are arranged in circumferential rows on the conical surface thereof at various distances from the heel surface. The row nearest, but not on the heel surface is known as the gage row.
In some types of bits, such as shown in U.S. Pat. No. 3,727,705, certain cutters have gage row structure that includes staggered rows located thereon. The staggered rows comprise two rows of inserts alternately spaced so that the grip portion of the inserts do not interfere. See also U.S. Pat. No. 4,343,372.
In other prior art bits (see FIG. 4), the inserts on the gage row are oriented in such a manner to cause the gage cutting elements to cut both the borehole bottom and sidewall. This combined cutting action compromises the insert because the cutting action operating on the borehole bottom is usually a crushing and gouging action while the cutting action operating on the sidewall is a scraping action. Ideally, a crushing action calls for a tough insert while a scraping action calls for a hard insert. One grade of tungsten carbide can not be hard and tough at the same time and can not ideally perform both functions. As a result, compromises are required and the gage cutters can not be as tough as the inner rows of cutters because they must be harder to accommodate the scraping action.
Other bits have a row of inserts mounted on the heel surface to contact the sidewall. However, this is usually accomplished after the gage row of inserts has formed the borehole sidewall and bottom.
U.S. Pat. Nos. 2,774,570 and 2,774,571 show such arrangements. In each instance, the gage row of inserts is still physically compromised because the lower portion of each insert must still engage the borehole bottom while the outside portion scrapes the sidewall. The heel row functions to maintain the gage after it is formed by the gage cutters. This is because the heel row cutters are separated a distance from the gage row cutters, and do not extend to the hole bottom.